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P-wave relative teleseismic residuals were measured for a network of seismological stations along a 300 km profile across the Adamawa Plateau and the Central African Shear Zone of central Cameroon, t o determine the variation in crust and upper mantle velocity associated with these structures. A plot of the mean relative residuals for the stations shows a long wavelength (> 300 km) variation of amplitude 0.45 s. The slowest arrivals are located over and just to the north, of the faulted northern margin of the Adamawa Plateau. The residuals do not correlate with topography, surface geology or the previously determined crustal structure, in any simple way.The Aki inversion technique has been used to invert the relative residuals into a 3-D model of velocity perturbations from a mean earth model. The results show the region is divided roughly into three blocks by two subvertical boundaries, striking ENE and traversing both the crust and upper mantle down to depths greater than 190 km. The central block, which is 2 per cent slower than the adjacent blocks, roughly corresponds to the Central African Shear Zone. The Adamawa Plateau, as an individual uplifted area, is explained by the interaction of a regional anomalous upper mantle associated with the West African Rift System, and the Central African Shear Zone, which provided a conduit for heat flow to the surface.

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